Objective lens for scanning optical disks (DVDS)

ABSTRACT

An objective lens for scanning DVD and HD-DVD optical disks, comprising a first part and a second part, wherein said first part is shaped to comprise a first surface and an opposing second surface, said second part is shaped to comprise a third surface and an opposing fourth surface, said first surface is of a convex shape providing a main part of the lens power, said third surface is shaped to fit said first surface, and wherein said fourth surface is a generally aspherical surface, said second part being formed of synthetic resin material and said first part being formed of a material having a refractive index n 1  having a value between 1.65 and 2, wherein the Abbe number V 1  of the first part and the Abbe number V 0  of the second part comply with the following relation (Formula I).

This invention relates to an optical element and an optical scanningdevice, and in particular but not exclusively to the scanning of opticalrecord carriers of different formats.

Generally, it would be desirable to provide an optical scanning device,and an objective lens therefor, capable of reading out from twodifferent relatively high density formats, i.e. higher density than theCD format.

Due to the introduction of the blue semiconductor laser (wavelength,λ=405 nm) new options to further increase the density on an optical diskcompared to the conventional Digital Versatile Disk CVD) (which uses ared laser, λ=660 nm) emerge. One option is the High Density-DVD (HD-DVD)system in which the wavelength is reduced from 660 nm to 405 nm but thethickness of the cover layer of the disk is the same as for the DVDsystem, namely 0.6 mm. A dual layer DVD can not readily be read using ablue laser. The reason for this is that the semi-reflective layer fordual layer DVD is made of gold, silicon and silicon carbide. For 650 nmthe reflectivity of both layers is ˜30%. For 405 nm the absorption ofgold becomes >50% and its reflectivity <10%.

It would be desirable to provide an optical scanning device whichincludes both a red laser and a blue laser. However, by changing thewavelength of radiation being used for read out, due to spherochromatismthe amount of spherical aberration generated by the objective in generalchanges and also the amount of spherical aberration introduced by thecover layer of the disk.

The paper “Objective lenses for red and blue lasers” by N. Murao et al.in the proceedings of the ODF2000 conference page 321–324, describesthree possible lenses for use in DVD and HD-DVD compatibility: (1) aglass doublet lens, (2) a photo-polymer doublet lens and (3) adiffractive optical element. A glass doublet lens is expensive to makeand therefore not a practical solution for mass production. Adiffractive optical element solution leads to losses in efficiencybecause in general it is not possible to realise such an element with100% efficiency in the designed diffraction order for both the DVDconfiguration and the HD-DVD configuration. Furthermore, the efficiencyloss increases due to manufacturing tolerances. A photo-polymer doubletlens as proposed includes a glass moulded aspherical lens with on theside facing the disk an additional photo-polymer material. A drawback ofthis solution is that one first has to make an aspherical lens and thenas a second step add the photo-polymer material, making the lens moreexpensive.

It would be desirable to provide an objective lens compatible with tworelatively high density formats, such as DVD and HD-DVD, withoutintroducing undesirable costs or undesirable losses in efficiency.

In accordance with the present invention there is provided an opticalelement comprising a first part and a second part, wherein said firstpart is shaped to comprise a first surface and an opposing secondsurface, said second part is shaped to comprise a third surface and anopposing fourth surface, said first surface is of a convex shapeproviding a main part of the lens power, said third surface is shaped tofit said first surface, and wherein said fourth surface is a generallyaspherical surface, said second part being formed of synthetic resinmaterial and said first part being formed of a material having arefractive index n₁ having a value between 1.65 and 2, wherein the Abbenumber V₁ of the first part and the Abbe number V₀ of the second partcomply with the following relation:$1.16 \leq \frac{V_{1}}{V_{0}} \leq {1.74.}$

The invention provides an objective capable of being used to read outdata from different optical record carrier formats, such as DVD andHD-DVD, using radiation of different wavelengths whilst providing forthe selection of materials for the objective capable of correcting forspherochromatism in the optical record carrier. Since only refractivesurfaces are involved the objective does not suffer from efficiencylosses as in the case of diffractive optical elements. Furthermore,since relatively few steps are needed to make the dual-compatible lens,it is a cost effective solution.

It would be possible to use a refractive single element objective madeof one material, in order for the amount of spherical aberrationgenerated by the objective to compensate the amount of sphericalaberration generated by a cover layer of the disk, while the objectivealso fulfils the Abbe Sine condition. However, then the refractive indexof the lens material would need to be larger than 2. Materials withrefractive index larger than 2 tend to be expensive and have a low Abbenumber, thus being sensitive to small wavelength variations.

The objects, advantages and features of the invention will be apparentfrom the following more particular description of preferred embodimentsof the invention, as illustrated in the accompanying drawings, in which

FIG. 1 shows a schematic cross-section of an objective lens inaccordance with an embodiment of the invention; and

FIG. 2 shows a schematic cross-section of an optical scanning device inaccordance with an embodiment of the invention.

DESCRIPTION

FIG. 1 shows an objective lens OL in accordance with one embodiment ofthe invention. The aspherical objective OL is formed from aplano-spherical glass body A with a thin aspherical correction layermade from a synthetic resin material shaped to fit and bonded to theconvex surface of the glass body. The lens may be produced in a mannersimilar to that discussed in U.S. Pat. No. 4,623,496, the contents ofwhich are incorporated herein by reference.

In the following, the design of the lens parameters is described indetail.

A third order Seidel analysis of the wavefront aberrations of the lenscan be made. Let the resin have refractive index n₀, Abbe number V₀,thickness on the optical axis d₀, curvature c₀ (power K₀) and fourthorder coefficient G determining the deviation from the spherical shapeof the layer. The plano-spherical glass body (flat surface facing thedisk) has refractive index n₁, Abbe number V₁, the thickness d₁ andcurvature c₁ (power K₁) while the disk has refractive index n₃, Abbenumber V₃ and the thickness d₃. The free working distance, hence thedistance between the lens and the disk, is d₂.

Since the aspheric layer is thin the following approximations can bemade:d₀≈0c₀≈c₁=cK=K ₀ +K ₁≈(n ₁−1)c

Using these approximations the third order Seidel term for sphericalaberration S_(I) is found to be: $\begin{matrix}{S_{I} = {{\frac{{NA}^{4}}{\left( {n_{1} - 1} \right)^{4}c}\left( {\frac{8{G\left( {n_{0} - 1} \right)}}{c^{3}} + {\left( {n_{1} - 1} \right)\left( {n_{1} - {2n_{1}} + \frac{2}{n_{1}}} \right)}} \right)} +}} \\{{{NA}^{4}\frac{d_{1}}{n_{1}}\left( {\frac{1}{n_{1}^{2}} - 1} \right)\mspace{14mu}\ldots} + {{NA}^{4}\frac{d_{3}}{n_{3}}\left( {\frac{1}{n_{3}^{2}} - 1} \right)}}\end{matrix}$while the term for coma aberration is given by: $\begin{matrix}{S_{II} = {{NA}^{3}\eta}} \\{\left\lbrack {\frac{1 - {n_{1}\left( {n_{1} - 1} \right)}}{n_{1}\left( {n_{1} - 1} \right)} + {\left( {n_{1} - 1} \right){c\left( {{\frac{d_{1}}{n_{1}}\left( {1 - \frac{1}{n_{1}^{2\;}}} \right)} + {\frac{d_{3}}{n_{3}}\left( {1 - \frac{1}{n_{3}^{2}}} \right)}} \right)}}} \right\rbrack}\end{matrix}$where the focus relation is given by:${\frac{d_{1}}{n_{1}} + d_{2} + \frac{d_{3}}{n_{3}}} = \frac{1}{K}$

In order for the lens solutions not to introduce spherical aberrationfor either the DVD case or for the HD-DVD case and to comply with theAbbe Sine condition the coma term should be zero for the DVD case. Hencethe solutions should fulfill the conditions:

-   -   S_(I)(660 nm)=0    -   S_(I)(405 nm)=0    -   S_(II)(660 nm)=0

From S_(II)(660 nm)=0 it follows that, in order to have d₁>0, therefractive index n₁ should fulfill the relation:$n_{1} = {\frac{1}{2} + {\frac{1}{2}\sqrt{1 + \frac{4}{1 - {\frac{d_{3}}{n_{3}}\left( {1 - \frac{1}{n_{3}^{2}}} \right)K}}}}}$

Inserting the values for a disk: d₃=0.6 mm, n₃=1.58 and, using a typicalvalue K˜0.35 mm⁻¹, we find that n₁>1.65.

Note that when solving S_(I)(660 nm)=0, S_(I)(405 nm)=0 and S_(II)(660nm)=0 while taking n₀=n₁ and V₀=V₁ we find that n₁>2.0 which is not apractical solution.

The following relates to the case where the aspheric layer and the glassbody have different properties as per the present invention. Thefollowing approximate relation between the Abbe number and the actualdifference between the refractive index at 660 nm and 405 nm holds:${\Delta\; n} \equiv {{n\left( {405\mspace{14mu}{nm}} \right)} - {n\left( {660\mspace{14mu}{nm}} \right)}} \approx {2\frac{n - 1}{V}}$

Using the above relation we find from S_(I)(660 nm)=0, S_(I)(405 nm)=0and S_(II)(660 nm)=0 after eliminating G and d₁ the following relationfor n₁: $\begin{matrix}{\frac{F}{{V_{0}\left( {n_{1} - 1} \right)}^{2}n_{1}} = {{\frac{d_{3}\left( {n_{3} - 1} \right)}{n_{3}^{2}V_{3}}\left( {1 - \frac{3}{n_{3}^{2}}} \right)\mspace{14mu}\ldots} -}} \\{\frac{\begin{matrix}\left\lbrack {{\left( {1 + {2n_{1}} - {5n_{1}^{2}} - n_{1}^{3} + n_{1}^{4}} \right)F} +} \right. \\\left. {\frac{d_{3}}{n_{3}}\left( {1 - \frac{1}{n_{3}^{2}}} \right){n_{1}\left( {n_{1} - 1} \right)}^{2}\left( {n_{1}^{2} - 3} \right)} \right\rbrack\end{matrix}}{\left( {n_{1} - 1} \right)^{2}{n_{1}^{2}\left( {n_{1} + 1} \right)}V_{3}}}\end{matrix}$where F=1/K. Note that this equation no longer contains n₀. Solving theabove equation using d₃=0.6 mm and, typically, n₃=1.58 and V₃=30 whiletaking F˜3 mm we find that in order to have 1.65<n₁<2.0 that:$1.16 \leq \frac{V_{1}}{V_{0}} \leq 1.74$$2.7 \leq {n_{1} + {0.7\frac{V_{1}}{V_{0}}}} \leq 2.9$

Since this is only a third order calculation the results are only afirst estimation. Therefore, ray-tracing may be used in order to improvethese results. We investigate the case where F=2.75 mm and the numericalaperture of the system is NA=0.65 for the DVD system. In building themerit function we used the wavefront aberration on axis and at 0.5°field for both the DVD and HD-DVD configuration. Table I shows theproperties of the materials, in one embodiment, of which the asphericlayer is made (Diacryl) and of the disk (Polycarbonate).

TABLE I n(660 nm) n(405 nm) V Diacryl 1.5640 1.5945 34.5 Polycarbonate1.5798 1.6188 29.9

Table II shows the parameters are listed defining the objective lens, infour different exemplary glass types, while Table III shows thecorresponding wavefront aberrations for the four examples.

The rotational symmetric shape can be described by the equation${z(r)} = {\frac{c_{0}r^{2}}{1 + \sqrt{1 - {c_{0}^{2}r^{2}}}} + B_{4} + B_{6} + B_{8} + B_{10} + B_{12} + B_{14}}$with z being the position of the surface in the direction of the opticalaxis in millimetres, r the distance to the optical axis in millimetres,and B_(k) the coefficient of the k-th power of r. The values of B₄ toB₁₄ for the surface of the objective lens facing the radiation sourceare tabulated in Table II for all the four cases.

TABLE II Example 1 2 3 4 Glass type LAK10 LAFN28 S-LAH59 LASFN31(Schott) (Schott) (Ohara) (Schott) n₁(660 nm) 1.71548 1.76822 1.810511.87400 n₁(405 nm) 1.74436 1.79976 1.84608 1.91811 V₁ 50.4 49.6 46.641.0 d₀ (mm) 0.027 0.019 0.021 0.020 d₁ (mm) 1.911 2.281 2.6528 3.1158c₀ (mm⁻¹) 0.5089 0.4804 0.4577 0.4258 c₁ (mm⁻¹) 0.4545 0.4367 0.41320.3861 B₄ (mm⁻³) −0.0038692065 −0.0077669621 −0.007358103 −0.0061609467B₆ (mm⁻⁵) −0.013928053 −0.0034012771 −0.0015370307 −0.00077627718 B₈(mm⁻⁷) 0.011563615 0.0015918806 0.000077327348 −0.0002798011 B₁₀ (mm⁻⁹)−0.0064570315 −0.0010955801 −0.0002126324 0.00004604568 B₁₂ (mm⁻¹¹)0.0017295657 0.00029753255 0.000057256433 −0.000013221979 B₁₄ (mm⁻¹³)−0.0001972081 −0.00003802723 −9.3622897 10⁻⁶ 8.9758754 10⁻⁸

TABLE III Example 1 2 3 4 On axis 23.8 mλ 18.9 mλ 17.9 mλ 18.5 mλ (660nm) 0.5° field 38.3 mλ 30.4 mλ 30.7 mλ 31.6 mλ (660 nm) On axis 26.7 mλ11.8 mλ 11.1 mλ 11.2 mλ (405 nm) 0.5° field 51.2 mλ 48.0 mλ 39.8 mλ 35.2mλ (405 nm)

From the ray-trace results, it was derived that plano-aspheric glassreplica lenses suitable for reading/writing DVD (660 nm) andreading/writing HD-DVD (405 nm) should preferably comply with therelations: n₁ > 1.65 $1.16 \leq \frac{V_{1}}{V_{0}} \leq 1.74$$2.46 \leq {n_{1} + {0.55\frac{V_{1}}{V_{0}}}} \leq 2.66$9.96 ≤ 6.99n₁ + d₁ ≤ 10.32 with  d₁  expressed  in  millimeters

Due to the higher order terms the results deviate slightly from theresults obtained from the third order analysis.

Reference is now made to FIG. 2. In accordance with embodiments of theinvention, including in a first specific embodiment the DVD and HD-DVDcase, different formats of optical disk, may be written and/or read-outby means of an optical pickup unit (OPU) such as that shown. The opticalcomponents of the OPU are held in a rigid housing which is formed ofmoulded aluminium or suchlike. The OPU is arranged in an opticalrecording and/or playback device such that the OPU travels along alinear bearing arranged radially of the disk during scanning of thedisk. Each disk to be scanned is located in a planar scanning areaadjacent to the OPU, mounted on a motorised rotating bearing in theplayback and/or recording device, whereby the disk is moved relative tothe OPU during playback and/or writing.

Each of the different formats of disk to be scanned by the deviceincludes at least one information layer. Information may be stored inthe information layer or layers of the optical disk in the form ofoptically detectable marks arranged in substantially parallel,concentric or spiral tracks. The marks may be in any optically readableform, for example in the form of pits or areas with a reflectioncoefficient different from their surroundings.

The OPU includes two optical branches for scanning disks with radiationof two different wavelengths, in this embodiment a wavelength ofapproximately 650 nm (referred to herein as “the red wavelength”) and awavelength of approximately 405 nm (referred to herein as “the bluewavelength”). It should however be appreciated that optical scanningdevices in accordance with different embodiments of the invention mayoperate at other wavelengths, and with more than two wavelengths.

A first optical branch which includes a polarised radiation source 2,for example a semiconductor laser, operating at a predeterminedwavelength, in this example the wavelength, to produce a first beam 4.The first branch further includes a polarising beam splitter 6 forreflecting the returned beam towards a detector system 8, and acollimator lens 10 for producing a more collimated beam.

A dichroic beam splitter 12 reflects the first beam 4 towards theoptical disk OD. In the optical path portion between the beam splitter12 and the optical disk OD, which portion is shared by the two radiationbeams of the device, lie a quarter wavelength plate 14, operative atboth the red wavelength and the blue wavelength, a dichroic aperture,operative to reflect radiation at the red wavelength in an area outsidea predetermined radial distance from the optical axis, and a dual beamobjective lens OL in accordance with the invention. The dual beamobjective lens is adapted for correctly focusing, with limited sphericalaberration, the collimated red wavelength beam to a spot on theinformation layer in a disk, such as a dual-layer DVD disk, operative atthe red wavelength, and a collimated blue wavelength beam to a spot onan information layer in a disk, such as an HD-DVD disk, operative at theblue wavelength.

The first beam is altered in polarisation from linear to circularpolarisation by quarter wave plate 14 and focused by objective lens OLto a spot on the disk OD. The reflected beam follows a return path,being transformed back to a beam exhibiting linear polarisationperpendicular to the incident beam by the quarter wave plate 14, and isreflected by beam splitter 6 towards a photodiode detector arrayarranged in detector system 8, where the data, focus error and trackingerror signals are detected. The objective lens OL is driven by servosignals derived from the focus error signal to maintain the focussedstate of the spot on the optical disk OD and from the tracking errorsignal to maintain alignment with a track on the disk OD currently beingread.

The second optical branch in this embodiment includes a polarisedradiation source 16, for example a semiconductor laser, operating at apredetermined wavelength different to that of the first beam, in thisexample the blue wavelength, to produce a second beam 18. The opticalpath for the second beam includes polarising a beam splitter 20 forredirecting the return beam for focus and radial tracking error signalgeneration at a detector array system 22 and a collimator lens 24 forsubstantially collimating the second beam. The second beam istransmitted substantially fully by the dichroic mirror 12, is altered inpolarisation from linear to circular polarisation by quarter wave plate14, and focused to a spot on an information layer in the disk OD. Thereflected beam follows a return path, being transformed back to a beamexhibiting linear polarisation perpendicular to the incident beam by thequarter wave plate 14, and is reflected by beam splitter 20 towards aphotodiode detector array arranged in detector system 22, at which adata signal and tracking and focus error signals are detected. Theobjective lens OL is driven by servo signals derived from the focuserror signal to maintain the focussed state of the spot on the opticaldisk 10 and the detector array, and from the tracking error signals tomaintain alignment with the track currently being scanned.

The numeric aperture (NA) of the objective OL, as used in the case ofboth the red and blue wavelengths, is greater than 0.5, and morepreferably greater than 0.55. In one embodiment, an NA of 0.6 is used.In a different embodiment, an NA of 0.65 is used.

The above embodiments are to be understood as illustrative examples ofthe invention. Further embodiments of the invention are envisaged. It isto be understood that any feature described in relation to oneembodiment may also be used in other of the embodiments. Furthermore,equivalents and modifications not described above may also be employedwithout departing from the scope of the invention, which is defined inthe accompanying claims.

1. An optical element comprising a first part and a second part, whereinsaid first part is shaped to comprise a first surface and an opposingsecond surface, said second part is shaped to comprise a third surfaceand an opposing fourth surface, said first surface is of a convex shapeproviding a main part of the lens power, said third surface is shaped tofit said first surface, and wherein said fourth surface is a generallyaspherical surface, said second part being formed of synthetic resinmaterial and said first part being formed of a material having arefractive index n₁ having a value between 1.65 and 2, wherein the Abbenumber V₁ of the first part and the Abbe number V₀ of the second partcomply with the following relation:$1.16 \leq \frac{V_{1}}{V_{0}} \leq {1.74.}$
 2. An optical elementaccording to claim 1, wherein said first part is a plano-convex part. 3.An optical element according to claim 1, wherein said first surface isof a generally spherical shape.
 4. An optical element according to claim1, wherein said first part is formed of a glass material.
 5. An opticalelement according to claim 1, wherein the refractive index n₁ and theAbbe number V₁ of the first part and the Abbe number V₀ of the secondpart comply with the following relation:$2.46 \leq {n_{1} + {0.55\frac{V_{1}}{V_{0}}}} \leq {2.66.}$
 6. Anoptical element according to claim 1, wherein the refractive index n₁and the thickness d₁ of the first part comply with the followingrelation:9.96≦6.99n ₁ −d ₁≦10.32, with d₁ expressed in millimeters.
 7. An opticalscanning device comprising an optical element according to claim
 1. 8.An optical scanning device according to claim 7, wherein the element isa lens having a numeric aperture greater than 0.5.